1. Field of the Invention
The present general inventive concept relates to audio encoding and decoding, and more particularly, to a method and apparatus to quantize/dequantize frequency amplitude data and a method and apparatus to audio encode/decode using the method and apparatus to quantize/dequantize frequency amplitude data.
2. Description of the Related Art
With the diversification of application fields of audio communication and the improvement in transmission speeds of networks, there is a demand for high-quality audio communication. In order to meet this demand, the transmission of a wideband audio signal with a bandwidth of 0.3 kHz-7 kHz, which has superior performance compared to a conventional audio communication bandwidth of 0.3 kHz-3.4 kHz in various aspects, such as spontaneity and articulation, is required.
A packet switching network that transmits data in packet units may cause channel congestion, resulting in packet loss and audio quality degradation. In order to address this problem, a technique for concealing a damaged packet is widely used. However, this technique is not a perfect solution to the problem. Thus, wideband audio signal encoding/decoding techniques capable of effectively compressing a wideband audio signal and solving the channel congestion problem have been proposed.
The techniques that are currently being proposed can be classified as three types of techniques. A first technique compresses audio signals in a 0.3 kHz-7 kHz band at a certain time and restores the compressed audio signals. A second technique divides the audio signals in the 0.3 kHz-7 kHz band into audio signals in a 0.3 kHz-4 kHz band (i.e., a low band) and audio signals in a 4 kHz-7 kHz band (i.e., a high band), hierarchically compresses the audio signals, and restores the compressed audio signals. A third technique compresses audio signals in a 0.3 kHz-3.4 kHz band, restores the compressed audio signals, over-samples the restored audio signals to wideband audio signals in the 0.3 kHz-7 kHz band, obtains a wideband error signal between the wideband audio signals obtained by the over-sampling and the original wideband audio signals, and compresses the wideband error signal.
The second and third techniques are wideband audio encoding/decoding techniques using bandwidth scalability, which allow the optimal communication in a given environment by adjusting the number of levels or the amount of data transmitted from a network to a decoder according to data congestion.
In wideband audio encoding that divides audio signals in the 0.3 kHz-7 kHz band into audio signals in a 0.3 kHz-4 kHz band and audio signals in a 4 kHz-7 kHz band and hierarchically compresses the audio signals, the high-band audio signals in the 4 kHz-7 kHz band are encoded by a modulated lapped transform (MLT). FIG. 1 is a block diagram illustrating a high-band audio encoder using the MLT.
Referring to FIG. 1, in the high-band audio encoder, upon input of a high-band audio signal, an MLT unit 100 performs the MLT on the input high-band audio signal and extracts MLT coefficients. Magnitudes of the extracted MLT coefficients are output to a two-dimensional discrete cosine transform (2D-DCT) unit 110 and signs of the extracted MLT coefficients are output to a sign quantization unit 120.
The 2D-DCT unit 110 extracts 2D-DCT coefficients from the magnitudes of the MLT coefficients and outputs the extracted 2D-DCT coefficients to a DCT coefficient quantization unit 130. The DCT coefficient quantization unit 130 arranges the 2D-DCT coefficients having a 2D structure according to magnitude, the largest statistical magnitude coming first, quantizes the arranged magnitudes (vectors), and outputs codebook indices corresponding to the quantized vectors. The sign quantization unit 120 quantizes and outputs the signs of the MLT coefficients of large magnitudes. The output codebook indices and quantized signs are provided to a high-band audio decoder (not shown), at a decoding end.
However, high-band audio encoding using the MLT has a difficulty in high-quality audio restoration in a low-bitrate audio transmission and undergoes degradation in the performance of audio restoration at low bitrates.
In an attempt to address these problems, a high-band audio encoder using a harmonic coder has been proposed.
FIG. 2 is a block diagram illustrating the high-band audio encoder using the harmonic coder. Referring to FIG. 2, when a high-band audio signal is input, a harmonic peak detection unit 200 detects a harmonic peak of the input high-band audio signal and outputs an amplitude and phase of the high-band audio signal based on the detected harmonic peak.
An amplitude quantization unit 210 quantizes and outputs the amplitude of the input high-band audio signal. A phase quantization unit 220 quantizes and outputs the phase of the input high-band audio signal. The output quantized amplitude and phase are provided to a high-band audio decoder (not shown), at a decoding end.
The high-band audio encoding using the harmonic coder can reproduce a high-quality audio at a low bitrate and with low complexity, however, the high-band audio encoding is limited in supporting bandwidth scalability for the input high-band audio signal.
Wideband error audio encoding compresses audio signals in a 0.3 kHz-3.4 kHz band providing bandwidth scalability, restores the compressed audio signals, over-samples the restored audio signals to wideband audio signals, obtains a wideband error signal between the wideband audio signals obtained by the over-sampling and the original wideband audio signals, and compresses the wideband error signal. In the wideband error audio encoding, the wideband error signals in a 0.05 kHz-7 kHz band are encoded by a modified discrete cosine transform (MDCT). FIG. 3 is a block diagram illustrating a wideband error audio encoder using the MDCT.
Referring to FIG. 3, in the wideband error audio encoder, when a wideband audio signal is input, a down-sampling unit 300 obtains a signal that is down-sampled to a low-band audio signal and a low-band audio encoder 310 encodes the low-band audio signal. The encoded audio signal is restored to a wideband audio signal by an up-sampling unit 320. A subtraction unit 330 subtracts the restored wideband audio signal from the original audio signal (i.e., the input wideband audio signal) to generate a wideband error signal. The generated wideband error signal is input to an MDCT unit 340 where MDCT coefficients of the input wideband error signal are extracted. The extracted MDCT coefficients are split into separate frequency bands by a band splitter 350 and the split MDCT coefficients are normalized by a normalization unit 360. The normalized MDCT coefficients are quantized by a quantization unit 370, and thus codebook indices corresponding to the normalized MDCT coefficients are output. The output codebook indices are provided to a high-band audio decoder (not shown), at a decoding end.
However, the wideband error audio encoding using the MDCT, also has a difficulty in high-quality audio restoration in a low-bitrate audio transmission similar to when the MLT is used.